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Nov 2015 p13 q1
330
A line has equation \(y = 2x - 7\) and a curve has equation \(y = x^2 - 4x + c\), where \(c\) is a constant. Find the set of possible values of \(c\) for which the line does not intersect the curve.
Solution
To find when the line does not intersect the curve, set the equations equal to each other:
\(x^2 - 4x + c = 2x - 7\)
Rearrange to form a quadratic equation:
\(x^2 - 6x + c + 7 = 0\)
For the line not to intersect the curve, the quadratic equation must have no real roots. This occurs when the discriminant is less than zero:
\(b^2 - 4ac < 0\)
Here, \(a = 1\), \(b = -6\), and \(c = c + 7\). Calculate the discriminant: